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Editor's note: This month only – you can buy two of my favorite ebooks, The Creative Photographer and Square, for just $7 each! Limited time only – click the links to learn more. Thanks for reading, Andrew

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Before we look at ISO in digital photography, a little history. Prior to the creation of the ISO standard (and digital cameras), there were two ways to measure film speed – ASA and DIN.

ASA was the most commonly used and easiest to understand. It worked on a linear scale, which means that a film that was twice as sensitive to light as another had double the ASA number. That way it’s easy to see that a 400 ASA rated film is twice as sensitive to light as a 200 ASA film.

DIN, on the other hand, used a logarithmic scale that wasn’t as intuitive. For example, a 100 ASA film had a DIN of 21, a 200 ASA film a DIN of 24, a 400 ASA film a DIN of 27 and so on. Each increment of three indicates a doubling of the speed of the film.

In the end, DIN was replaced by ASA, which in turned was renamed to ISO, the international standard that all camera and film manufacturers use today.

If you want to learn more about DIN and ASA this Wikipedia article has all the detail.

Why do we use ISO to measure light sensitivity?

The reason that ISO works so well as a measure of light sensitivity is that it makes it easy to think of film speed or the the ISO settings on a digital camera in stops.

For example, if you change the aperture setting of your lens from f8 to f11, you halve the amount of light entering the lens.

That means you need to double the ISO in a digital camera to keep the exposure the same (assuming that the light levels and shutter speed are unchanged).

This calculation is so easy to make once you understand the basics of exposure you don’t even have to think about it.

What is ISO?

In (really) simple terms, ISO tells you how sensitive the camera’s sensor is to light.

At ISO 50, 100 or 200 your camera’s sensor is least sensitive to light. More light needs to reach it to get a good exposure.

At higher settings, like 6400 or 12800, the sensor is much more sensitive to light. You can get a good exposure with much less light.

That’s all you need to know in order to use ISO properly on your digital camera. But it’s more interesting to dig a little deeper into how it works.

How ISO really works

Your camera’s sensor has a set sensitivity to light. This is known as the camera’s base ISO, usually 100 or 200.

When you take a photo the sensor’s pixels measure the quantity of light that hits them. Your camera reads these measurements and turns them into a digital file.

If you set a higher ISO and leave the aperture and shutter speed unchanged (and the ambient light level stays the same!) then the amount of light reaching the sensor is also unchanged.

The difference is that your camera amplifies the measurements made by the sensor to compensate for the lower light levels. In other words, the sensor’s sensitivity to light stays the same, but your camera amplifies the signal to simulate the effect of increasing ISO.

Why is an ISO standard important?

We use an ISO standard because it’s important that photos taken with different camera and lens combinations all produce the same results, when it comes to exposure, at the same aperture, shutter speed, and ISO settings.

The ISO standard is set by the International Organization for Standardization. All camera manufacturers use this standard to measure and rate the sensitivity to light of their camera sensors.

Because of this a photo taken at ISO 200, f7.1 and 1/200th of a second, such as the one below shot with my Fujifilm X-T1, should look the same as one taken at the same settings on your camera.

In another scenario, imagine a studio photographer sets up some lights and uses a flash meter to measure that the required exposure is, say, f8 at ISO 100. It’s important to know that this exposure setting works for any film or digital camera.

In practice, there may be variations in the accuracy of ISO settings between different camera models. But with digital cameras at least, these are minor and nothing to worry about.

Now it’s time to take a closer look at ISO and what the choice of ISO setting means for your photos.

How to use ISO

ISO is part of the exposure triangle. The aperture and shutter speed settings determine how much light enters your camera’s lens and reaches the camera’s sensor. The ISO, for all practical purposes, determines how sensitive your sensor is to light. Your selected combination of all three settings determines whether your photo is accurately exposed or not.

ISO, aperture and shutter speed all work together. You can’t leave one out of them of your exposure calculations.

How to choose the best ISO setting for your camera

Generally speaking the best ISO setting to use, in terms of image quality, is your camera’s base ISO setting.

For example, I used ISO 100 for this landscape photo.

The thing is, there are three sides to the exposure triangle. You may want to use a small aperture to get lots of depth of field. You may be shooting in low light for the mood. In the photo above I set the aperture to f8 and needed a shutter speed of 30 seconds to get a good exposure.

That’s fine for landscape photography, but what if you’re hand-holding the camera? Then you need to set a certain shutter speed to prevent camera shake. Or what if you want to set the aperture to f8, but the light levels mean you can only use f2.8 at your camera’s base ISO?

For instance, in this photo I decided to use a shutter speed of 1/180 second and an aperture of f3.2. The low light (it was taken indoors in a dark building) meant I needed to use ISO 3200 for accurate exposure.

All factors need to be taken into consideration. Creative decisions like depth of field are as important as image quality. There are times when you have to use a higher ISO setting to get a good exposure.

ISO and noise

But raising ISO has side effects. It decreases dynamic range and increases noise. It’s not the only factor that effects noise (underexposure is another) but it’s a major one.

Luckily, thanks to the high ISO performance of digital cameras this isn’t the problem it once was (although you still need to be aware of it).

I remember having to think hard about whether to use ISO 400 instead of ISO 100 on my first digital SLR. Now you are more likely to be worrying about whether you should be using ISO 3200, 6400 or 12800.

The best approach is this. When image quality is a priority use your camera’s base ISO (100 or 200). If it’s not practical to use that ISO setting then use a higher one. Aim to keep it as low as possible but don’t worry too much about it. And always remember that depth of field and shutter speed are just as important as ISO.

Avoiding Auto ISO

Another benefit of using digital cameras is that most models have an Auto ISO setting. That’s where you let the camera decide what ISO to use rather than make the decision yourself.

There are a few reasons why I don’t like using Auto ISO (and hence don’t recommend it to other people).

I learned photography on film cameras so I’ve been trained to think of ISO as a a fixed quantity, not a variable one. That’s a hard habit to break.

My camera tends to err on the side of setting a high ISO. Yes, you can usually control this to a degree by setting the highest ISO setting your camera can use. But as you might need to change this depending on the situation it makes the business of setting ISO more complex than it needs to be. It’s easier to set ISO yourself and change it if you need to.

Finally, setting the ISO yourself makes you think about the relationship between ISO and image quality.

For example, let’s say you’re in a situation where you are hand-holding the camera and can’t use a slower shutter speed because of camera shake. But light levels have dropped and you need to adjust the exposure.

If you open the aperture, you’ll have less depth of field. If you raise the ISO, you’ll get more noise.

You have to make a choice. What’s more important, depth of field or noise? You get to make the decision. You have creative control, your camera doesn’t.

Here’s an example. I made the photo below indoors with a hand-held camera. I set the shutter speed 10 1/125th of a second to avoid camera shake. Then I decided to shoot at ISO 6400 and f5.6 to give good depth of field. Alternatively, I could have used settings of ISO 800 and f2 to make a photo with much less depth of field. The choice is yours!

Having said that, I understand that some photographers like to use Auto ISO. They find it useful when taking photos in situations that don’t give you much time to think about camera settings, such as taking photos of active kids.

So, feel free to experiment with Auto ISO. Learn how it works on your camera and make up your own mind about whether it’s useful for you.

Full-frame versus crop sensor and medium format cameras

Generally speaking the larger your camera’s sensor the better the image quality, especially when it comes to noise.

That’s why some photographers prefer to buy full-frame or even medium format cameras rather than cameras with APS-C or Micro Four-thirds sensors.

But, if you’re thinking about buying a new camera, don’t feel you have to buy a full-frame model.

In practice the image quality (especially regarding high ISO performance and noise) on crop sensor cameras is more than good enough for most photographers.

There are plenty of pro photographers that use APS-C or even Micro Four-thirds cameras. If the image quality is good enough for a pro, then it’s good enough for a hobbyist.

For example, when I tested my Fujifilm X-T1 camera, which I bought when it was Fujifilm’s newest model, against my older Canon EOS 5D Mark II I found that the image quality from the newer camera (APS-C sensor) visibly beat that of the older full-frame camera.

I’m not knocking Canon. The difference is due to advances in sensor technology. You can expect this trend to continue in coming years.

APS-C and Micro Four-thirds cameras have advantages in terms of size, weight and price over full-frame cameras. Noise and image quality are important, but they’re not the only factors to consider.

Further reading

This is part of a series of lessons about photographic exposure. You can read the other lessons using the links below.

• Exposure Lesson #1: How to Choose an Exposure Mode
• Exposure Lesson #2: Why Cameras Get Exposure Wrong
• Exposure Lesson #3: Does the Metering Mode Matter?
• Exposure Lesson #4: Manual Mode
• Exposure Lesson #5: How to Read a Camera Histogram
• Exposure Lesson #6: Exposing to the Right
• Exposure Lesson #7: Exposure Compensation Or Manual Mode?
• Exposure Lesson #8: How To Use Bulb Mode Successfully